Process and means for the production of vacua



' March 14, 1939. K. c. D. HI CKMAN 2,150,685

PROCESS AND MEANS FOR THE PRODUCTION OF VACUA Filed March 11, 1957 2Shets-Sheet 1 Kenneth CDHickman INVENTOR.

ATTORNEYS March 14,' 1939. K. c. D. HICKMAN 2,150,585

PROCESS AND MEANS FOR THE PRODUCTION-OF VACUA Filed March 11, 1957 2Shets-Shet 2 120. FIGZ.

Kenneth flDfi g gg ry a n B'Y W n/u/WMW ATTORNEYS Patented Mar. 14, 1939UNITED STATES PATENT rnocsss AND MEANS FOR, 'rnn raonuc'rrou or vacuaKenneth C. n. Hickman, Rochester, N. Y., aa-

signor, by 'mesne assignments, to Distillation Products, Inc.,Rochester, N. Y., a corporation of Delaware Application March 11, 1937,Serial No. 130,359

3 Claims.

ing was obtained with high backing pressures of thev order of 20 mm.-Ordinary mechanical vacuum pumps were entirely suitable as backing pumpsfor this reason. Organic pumping fluids require a very lowbackingpressure usually less I than .1 mm. and preferably as low as about .02mm. and below. While the condensation pumps using organic fluids willwork against a slightly higher backing pressure, their pumping actionbecomes exceedingly erratic with backing pressures of .1.2 mm. and atslightly lower pressures the pumping action is very .slow. At backingpressures of .3 mm. the pumping action ceases entirely. Thischaracteristic necessitates the use of specially constructed mechanicalpumps giving very low backing pressures with a great. loss inefliciency. These pumps do not have sufliciently high capacity forcommercial high vacuum distillations and a plurality must be usedfurther increasing the complexity and expense of operation. Anotherobjectionable feature of organic pump fluids is their tendency todecompose with ,1 overheating caused 'bybreaking the vacuum or stoppingof the pumping operation.

A further disadvantage ofsuch pumps, especially those of large size, isa tendency to boil dry, necessitating constant or periodic addition 40of fresh pumping fluid. This has been due in most cases to incompletecondensation or im=- proper selection of pumping and condensingconditions.. Another cause is that in heavy. duty pumpingj so much gaspasses through the pump that the operating vapor is carried away inthestream. When high boiler pressures are employed these effects areespecially noticeable-since the pump liquid is rapidly converted intomore volatile materials which are withdrawn without condensation.

This invention has for its object to overcome or minimize the abovedeficiencies of pumps filled with organic pumping fluids. Another objectis to providean improved method which will enable organic pump fluids tobe used with backing pressures obtainable with ordinary mechanical pumpsor high velocity steam ejectors. Another object is to provide a pumpusing organic liquids which will efliciently operate against a backingpremure considerably higher than that which has hereto- 6 .fore beennecessary with such pumps. A further object is to provide a multi-stagepump employing organic liquids which will give a vacuum as high as thatheretofore obtainable with organic fluids and which will'eflicientlyoperate against backing l0 pressures substantially in excess of .2 mm.without substantial decomposition. A further object is to provide amethod of heating the pump fluids which will avoid substantialdecomposition. Another object is to provide a pump in which 15 loss ofpumping fluid resulting in the pump boiler becoming dry, is avoided.Otherobjects will appear hereinafter.

These objects are accomplished by the herein described invention whichin its preferred 20' embodiment comprises employing considerably higherboiler pressures in an organic condensation pump than has heretoforebeen used in pumps of this type. Suflicient kinetic energy is thusimparted to the vapors oi the organic pumping fluid 25 I that a higherbacking pressure can be overcome.- Substantial decomposition at thehigher temperatures necessary to produce the higher pressure is avoidedby employing an organic pump fluid which has a higher vapor pressurethan true low 30 vapor pressure condensation pump fluids heretoforeused. Where suiliciently high vacua is not produced by such a pump itmay be used in series with a condensation pump employing a true lowvapor pressure organic pumpfluid. By operat- 35 ingin this manner,asteam ejector can be used for backing purposes resulting in greatlyincreased efliciency of the pumping unit. Loss of pumping fluidresulting in the pump boiling dry is prevented byfractionating the pumpvapors 40 allowing only the light ends to be removed by the backingpump. 7 Condensation pumps filled with organic pump fluids havebeenunsatisfactory against high backing pressures due to the ease withwhich such or- (5 ganic compounds are decomposed. Organic con-'densation pump fluids have a very low vapor presr sure and this factaccounts for the high vacuum which they, give as compared with mercurywhen cooled traps are not used. This same property is 5 also responsiblefor the ease of decomposition and inability to produce a high vaporpressure of pump fluid, especially at pressures easily obtained withordinary mechanical pumps or steam .ejectora' While a mercurycondensation pump will operu pressures.

ate against backing pressures of 15-20 mm. this is due to the fact thata high boiler vapor pressure of mercury such as 250 mm. can be usedwithout fear of decomposition. Any attempt to produce a boiler pressureapproaching this value with ordinary organic condensation pumps wouldresult in complete decomposition of the pumping fluid. As a resultorganic fluid pumps have been operated under very low vapor or boilerpressures and published-descriptions of their use are replete withwarnings to avoid vapor or boiler pressures above the extremely lowvalue of 1.5 cm. of the pumping fluid (about 1. mm. Hg.) in order toavoid decomposition. Many investigators have advocated boiler pressuresof below .5-1 cm. of pumping fluid as a maximum i'f decomposition is tobe avoided even with backing pressures as low as .02 mm. Hg.

Highervapor pressure pumping fluids can be used efliciently againstbacking pressures substantially in excess of. .2 mm. such as at .5- mm.if the organic vapors are generated in the boiler under high vaporpressure. Sufiicient kinetic energy is thus imparted to the pump vaporsto enable working against the higher backing Although the increasedboiler pressures slightly decrease the degree of vacuum obtainable ithas been found that at the higher boiler pressures, the speed of thepump is not only unimpaired, but is actually substantially increased.This is possibly due to the fact that the pump is handling gases whichare at a pressure intermediate between that at which the Knudsen type offlow and the Poiseuille type of flow takes place.

While such a pump will not give as low a pressure as the best organiccondensation pumps the benefits of both may be utilized if desired byconnecting them in series Thus a condensation pump using octyl phthalatewill give a vac- 4 uum as high as 10- mm., but will operate withefilciency and without decomposition only against backing pressuressubstantially below .1 mm.

However, if a booster pump employing a high vapor pressure of pumpingfluid is interposed between the octyl phthalate pump. and a backingpump, eflicient evacuation to 10 mm. or below will be obtained withconsiderably higher backing pressures. Backing pumps capacity such asordinary mechanical pumps can thus be employed with organic pump fluidswhere they were previously unsuitable. Also high velocity multi-stagesteam ejectors which are .known to be very efficient can be used for thesame purpose.

Organic fluids which maybeemployed as operating fluids in the boosterpump should be capable of withstanding vaporization under the relativelyhigh boiler pressures without substantial decomposition. Compoundshaving a vapor pressure considerably higher than that of the pump fluidin the flne "pump are subject to less decomposition'and can be vaporizedat higher pressures and maintained at high boiler pressures withoutsubstantial loss. It has been found that pump fluids having a vaporpressure the same as or higher than di-amyl phthalate may be used in thebooster pump. Such compounds can be vaporized without decomposition atsuch a temperature that their vapor pressures are sufficiently high tooperate against high backing pressures.

Since the backing pump pressure must'be less than the boiler pressure ofthe booster .pump, it is apparent that the selection of the-particularsebacates, squalene and having high booster pump liquid will depend uponthe backing pressure against which it is to operate. As a general ruleit may be stated that the lower the vapor pressure of pump fluid thelower must be the backing pressure. Amyl phthalate represents the lowestvapor pressure which may be employed, and it therefore requires a lowbacking pressure of below-5 mm. and preferably of the order of about .4to 1 mm. for efficient pumping. Although backing pressures within thisrange can be efliciently obtained with steam electors or high capacitysingle mechanical pumps I prefer .to employ fluids which have a highervapor pressure than amyl phthalate in order to avoid all possibility ofdecomposition at these or higher backing pressures. Examples of suchfluids are petroleum distillates having a vapor pressure not greaterthan .1 mm. at 20 C. or less than .1 mm. at 160 C., dimethyl, ethyl,propyl, and butyl phthalates, diglycerol tetrapropionate, tripropionin,tributyrin, triamylin, amyl naphthalene, dimethyl and ethyl thehydrocarbon obtained by the molecular distillation of a fish oildisclosed in U. S. application #84370 filed June 12, 1936, by J. G.Baxter. Such substances will operate against higher backing pressuresand will withstand decomposition at the higher boiler pressures andtemperatures. Any other organic fluid can be used which has the requiredvapor pressure and stability.

In order to retain a pumping fluid of constant composition and alsoavoid loss of pumping fluid resulting in a dry boiler, it is a distinctadvantage to place a fractionating condenser between the booster pumpand the backing pump. In this way all useful components can be condensedand returned to the boiler of the pump and light ends can be segregatedand either allowed to be drawn into the backing pump or retained in thefractionation -device. A separate boiler can be used for this purpose orthe boiler of the pump itself can be made to supply the vapors to thefractionator. Where several pumps are used in series, each beingprovided with a different vapor pressure fluid, contamination of onepump fluid with another can be prevented by providing each pump with afractionating device. The volatiles will. thus be segregated and thrustback to the higher vapor pressure pump from which they came. For a moredetailed description of the construction and mode of operation of suchdevices, reference is made to my co-pending application #2'7,652 filedJune 21,1935.

With the foregoing explanation the principles of my invention will bemore fully understood by reference to the accompanying drawings inwhich:

Fig. 1 illustrates in diagrammatic form an elevation in section of anassembly of a series .of condensation and ejector pumps embodying theprinciples of my invention and:

Fig. 2 illustrates a sectional elevation of a preferred form of boosteror condensation pump provided with a fractionating device.

Referring to Fig. 1 reference numeral l designates a curved conduitconnected at one end.

to a chamber to be evacuated and at the other end to conduit 2 whichcommunicates with the intake chamber or low vacuum side of booster pumpB. Reference character A designates a condensation pump mounted upon thebottom inside wall at the lowest portion of the curve of conduit I,having an opening 3 in the side thereof and a flared jet 4, and numeralIi designates portion 8. Surrounding the nozzle is a chamber 9communicating with conduit 2 and with condensing tube i8 provided withexternal cooling jacket il through which cooling fluid is circulated bymeans of conduits l2 and I8. Pump l4 serves to return pumping fluidcondensed .in i8 to boiler 5. Conduit 15 connects the'lower portionofcondensing member N with the intake or low pressure side of highvelocity steam ejector pump C the high pressure side ofwhich isconnected to the intake chamber oi steam ejector D by conduit 20. Steamejectorpump E withdraws gases from the high pressure side of pump Dthrough conduit 2! and ejects them through exhaust 22 which is atatmosphericpressure.

Super-heated steam for operation of ejector pumps 0; D and E and forheating condensation pump 13 is admitted into the system through valve22 and conduit 23. Pumping fluidin boiler 16 of pump A is vaporized byan external packed heating jacket 24 provided with electrical resistancecoil '25. Boiler 5 of pumpB.,is heated by steam passed through aspiral-shaped conduit 28 connected to 23 by means of conduit 28 providedwith valve 88. Exhaust steam is with-' drawn through conduit 3l-andpreferably delivered to a suitable heat regenerator. Since the detailsof construction 'of multi-stage high velocity steam ejectors do not forma part of this .invention and are well known in the art, only a generalillustration of their construction mode of operation have been given.

' Referring to Fig. 2 reference numeral 58 designates thecylindricalcasing of .a booster pump which is closed atthe base by integral plate5| and at the top by plate 52 whichmakes a gas tight joint with flange53 by interposing a gasket or suitable sealing compound. The base ofcylinder, 58 is cut out to form supporting legs 54 and ' and openings 55are provided for exitof combustion gases from burner 58 which issupplied with combustible fuel through conduit 51. A gas intake conduit58 is located at the upper end of 58 and is connectedto a condensation,pump or the chamber to be evacuated. "A concentric cylinder 58 islocated inside casing 58 and is maintained in the positioni;shown bytheflange 68- which is welded to theinternaltwalls of the casing. NumeralBl *designates a body of pumping fluid which is located ,in the boiler62 and numeral 83 a conduit connecting the space above flange 68 withthe boiler, and terminating below the surface of fluid 8|. A rod 84 ispassed through a centrally located packed gland and is attached at thelower end to a three leggedcentering 'spider It, the legs of which touchthe insidewalls of 59. but are free to move inla vertical direction. Acylindrical false top. H is fastened to rod 64 by' 'means of integralplate I2 in such a manner that it is concentric with 58 a and 58. Theperiphery of II is flared as shown to form an annular jet 14 similar tothose of the well-known umbrella type of condensation pump.

Plate 12 is provided with a plurality of openings 13 as shown. 'Atube-l5 is centered around rod 88 and is threaded at the top and mountedin gland I6 in the same mannergas rod 84. The lower end of 15' isattached to flange ll which is flared downward at. the'periphery to'forman annular Jet 18. The tops of rod 84 and tube I5 .are provided withknurled knobs l8 and so that they can beturned to adjust the openings ofjets I4 and 18. To prevent entry of gas glands and 15 are provided withreservoirs 8| and 82 which are fllled with a low vapor pressure sealingliquid. I

A condensing element 83 is located on the outside of so and integraltherewith, cooling fluid the internal walls of conduit 85 and serve tofractionate pumping vapors as they are condensed. The-top plate isprovided with a withdrawal conduit 92' for removal of the lightest ends.Most of the vapors are supplied to elements 8| .by a boiler 93 connectedby the top.

plate 88 to conduit 95-the end of which terminates near the lowestfractionating tray 9|. The base of boiler 93 is shaped similar to thatof 52 and is provided with a burner 98. The base plate 91 is at aslightly lower level than that of plate 5i sothat fluid 6| flows at aslow rate through conduit 98 provided with screen liil and into 99.Removal or addition of fluid is accomplished by introduction throughconduit I08. A conduit H0 communicates at its uper end with the lowesttray 9i and at its lower end with conduit 88. It isprovided with a dropcounter ill in which is located a glass window 2. A glass tube H5connects conduit 58 with the lower and upper portions of boiler 82 bybranched conduits I I6 and I I1. This serves as a manometer indicatingthe pressure in the boiler as well as the height of the pump liquid. Aseries of baflles 8 are mounted upon shaft I!!! which is-centrallylocated in conduit 85 by spiders I20 and, Hi.

These baiiies ,prevent the withdrawal of pump a temperature of about C.is admitted through valve 22 and the system evacuated by steam ejectorsC, D and E. Valve 30 is then opened and the pump fluid in B heated tovaporizing temperature under a boiler pressure considerably in excess of.3 mm. Hg. Vapors pass through conduit 8 and restriction 8 and expand ina relatively high velocityjet into condensing chamber I 0. Gasesarriving through conduit 2 are entrained'in the jet and forced intoconduit I5 from which they are removed by the steam ejector pumps.Liquid condensed in i8 flows to pump i4- and is returned to boiler 5.Pump B when filled with di methyl phthalate and heated 'sufllciently togive a boiler pressure of 5-25 cm. of pump fluid will rapidly produce avacuum of below .05 mm. against a backing pressure of between .5 and 10mm. Hg. After pump B has been inoperation and reduced the pressure ofthe system to substantially below .1 mm., pump A is started byelectrical heating unit 25. A low vapor pressure pump fluid such asdi-octyl phthalate or Apieson oil is employed in pump A. Vapors frompump A flow through jet 4 and are condensed on walls of conduit I,condensed liquid flows down the bottom of conduit i and again enters theboiler it through opening 3. To prevent loss of vapors through thisopening the pump fluid level should be above the highest point of 3. Therate of vaporization and temperature of pump fluid in boiler 5 iscontrolled by the amount of steam flowing through valve 30. Finalpressures of about 10-. min. and below can be efficiently produced bythis system.

In operating the apparatus of Fig. 2 the boilers 82 and 99 are filled,to about the level indicated, with a suitablebooster pump fluid such asbutyl phthalate; The conduit 58 is connected to the chamber to beevacuated, or to the high pressure side of a. condensation pump if it isto be used in series, and conduit 81 is connected to a backing pumpwhich is put into operation. After the pressure in the system has beenreduced by the backing pump, burner 58 is started and the butylphthalate vaporized under a boiler pressure of 4 to 12 cm. head of butylphthalate. The vapors travel up through 59, part of them enter theopenings 13 and flow in a reversed direction through annular jet I8. Thebalance of the vapors flow through jet 14. The vapors are thus expelledas a vigorous double jet into the passage formed between cylinders 50and 59. Gases flowing through conduit 58 are entrained in the jet andforced downward into conduit 84 and are withdrawn by the backing pumpconnected to 81. The heavier vapors are condensed by contact with thecooled walls in the vicinity of cooler 83 and flow down the walls of 50onto flange 60 and are returned to the boiler by conduit 63. The optimumjet openings are obtained by varying the distances by means ofadjustment knobs l9 and Bil.

During the pumping action burner 96 is started and the pump fluid in 99vaporized. The vapors travel up through conduit 95 and are condensed in85. Due to the fractionation effect light ends condense in the top ringsand can be withdrawn through conduit 92. Heavier components arefractionated and those free of light materials flow from the lowest ring9i into conduit H and back into the boiler of 62 via conduit 63. Duringthe functioning of boiler 99 fresh fluid flows from 62 into 99 by way ofconduit 98. The number of rings 9| and the degree of cooling at 88 canbe varied greatly to separate any desired range of volatile materials.The pump fluid is therefore easily maintained at. any predeterminedcomposition. A particular advantage of the fractionation construction isthat loss of pump fluid resulting in a dry boiler and destruction of thevacuum is prevented. Pumps of this type can be operated continuously formonths which would be impossible with old types of construction.Pressure in boiler 62 is adjusted by varying the openings of the, jetsand the amount of heat supplied.

Many changes can be made in the apparatus illustrated and in its mode ofoperation without departing from the spirit or scope of my invention. Itis possible, for instance, to place a trap containing a hygroscopicagent such as calcium chloride or sodium hydroxide, between the steamejector and the booster pump, thus avoiding any possibility of watervapor traveling into the latter. Instead of using one or two boosterpumps any number can be used in series to give a gradually increasingdegree of vacuum. Obviously if the booster pump gives a vacuumsufflciently high for the particular purpose it need not be used incombination with a high vacuum condensaarsaoas tion pump. The use offractionating devices is preferred, but the pumps will work withoutthem.Baflles H8 of Fig. 2 can be eliminated if the volume of gas removed isinsufllcient to carry material amounts of pump fluid vapor into thebacking pump.

The essential distinction between booster pumps as described andcondensation pumps heretofore known is inthe pump fluid employed, themethod of operating the pump and the use of fractionating expedients.The construction must be changed somewhat from that of known pumps toenable higher pressures to be developed in the boiler and to preventback passage of vapors into the high vacuum side, but they are otherwisemuch the same in detail of construction and the booster pumps can bemade in a variety of forms now common to condensation pumps such asvertical or inverted jet, mushroom and multiple-jet mushroom types. Inorder to generate the pump vapors a restricted jet or a divergent orexpansion type nozzle or jet with restricted throat is used thuspreventing the generated vapors from escaping as fast as formed. Thevapors are thus generated 'under higher pressure and escape through theexpansion nozzle as a vigorous vapor stream. Due to the high velocity ofthe jet. proper selection of the expansion ratio and the use of smallclearance, the back passage of vapors into the chamber to be evacuatedor into the condensation pump is prevented. These details ofconstruction, as is well known, vary according to the size of the pump,the particular fluid used, boiler pressure etc. and cannot therefore beempirically defined. However the construction in each particular casewill be obvious to those skilled in the art of nozzle and jet pumpdesign.

As indicated above amyl phthalate having a vapor pressure of about 2x10"mm. at 20 C. represents the lowest vapor pressure which a booster pumpfluid may have if decomposition is to be substantially avoided. Theupper vapor pressure limit depends entirely upon what vacuum it isdesired to produce. If a vacuum of .05 is required, a pump fluid havinga vapor pressure at the temperature of the pump condenser of .05 mm. orbelow is required. .Fluids having the required stability and vaporpressures of between about .05 and about 2 0- mm. at room aresatisfactory, those having a vapor pressure of between about'.01 and2x10- mm. being most generally useful. The boiler pressures used can beso high that they approach the decomposition point of the liquid used.For liquids having a vapor pressure in the higher portion of the aboverange, boiler pressures of up to 20 mm. Hg. are usually mostsatisfactory. Boiler pressures of up to 15 mm. such as between and2.5mm. Hg.

are generally used, the lower values being best when using liquidshaving the lower vapor pressures. The booster pumps will operateefliciently against backing pressures substantially above .3

mm. such as .5-10 mm. These pressures can easily be obtained withordinary mechanical pumps or steam ejectors. Backing pressures of .5-2mm. are desirable with the lower vapor pressure liquids especially whenused with large pumps designed for high capacity, but higher pressurescan be used with efflciency.

While I prefer to use steam or other heated vvapor as a means of heatingthe lighter or higher vapor pressure pump fluids it is apparent thatother heating means such as electricity can be used. Vapors such assteam have the distinct tained when the heating fluid is shut oil. Thisfactor is of considerable importance in preventing destruction oi thepumping fluid when thecomprising in combination a condensation pumpvacuum is broken due to accident or trouble with backing pumps. Alsooverheating will be avoided and rapid starting, therefore possible dueto the fact that no temperature above that of the steam can .be appliedto the pump fluid.

' Although I have employed the terms,- high pressure, relatively highboiler pressure. etc,, it is to be understood that the invention relatesto vacuum and especially highvacuum production and that theseexpressions have the meaning which, would generally be given them inthis art.

In the claims all pressures are mercury. i I

What I claim is: 1. Apparatus for exhausting'ciosed receptaclescomprising in combination, a condensation pumpfiiledwithanorganicpumpfluidhavingavapor in millimeters of I pump.

pressurebetween .05 and 2x10 mm. at 20' C; and a high velocity steamejector connected on the low vacuum side.

2. Apparatus for exhaus ing closed receptacles the boiler oi which isiilled with an organic pump fluid having a vapor pressure between .01and 2x10 mm. at 20 C. and a high velocity steam ejector connected on thelow vacuum side of the 3. Pumping apparatus for exhausting a closedreceptacle-comprising in combination a condensation pump the boiler ofwhich is filled with an organic pump fluid having a vapor pressure'between .05 and 2x10 mm.-at 20 C, and a high velocity steam ejectorconnected to the low vacuum side of the condensation pump thecondensation pump being provided with iractionat-- in: means forseparating the volatile constituents irom the pump fluid.

mm C. D. HICKMAN.

